How Spiders Keep The Peace

Credit: Francis Ratnieks

By Nisha Giridharan

The Parawixia bistriata (pictured), a large spider that lives in the semiarid habitats of southern Brazil, knows that when it comes to living in a crowded neighborhood, fighting is never the answer. 

Scientists knew that communities of P. bistriata emerge from collective shelters at sunset and build individual webs along permanent and communal silk support lines. 

At the end of the night, however, a small number of webless spiders peacefully share the prey of their neighbors—a rare instance of cooperation among arachnids. 

Now, after months of observation, researchers have developed a new model explaining the behavioral strategies of these webless spiders. 

Once the sun begins to set, P. bistriata swarm the communal silk lines, searching for spots to build their webs. As each spider claims its territory, it shoos away others that try to settle uncomfortably close by. Early in the evening, the still-webless interlopers graciously move on, knowing that they will likely find an empty spot elsewhere. But as the night progresses, desperation sets in. Open sites become scarce, so webless spiders no longer retreat when resident spiders bounce up and down to drive them away. Instead, they sit near a resident's web and freeload on large prey—seemingly with the resident's permission. 

Applying insights from game theory, the researchers concluded that resident spiders don't aggressively defend their territory because a night's meal is not worth risking their lives in a fight, they report this month in The American Naturalist. 

What's more, any spider could find itself on the other side of the negotiation the next night, when the race for space begins again.